The present invention generally relates to a method & design of cutting and cutting rotative bits, which can be used for excavation, planing and drilling of rock and soil and other non-metallic brittle materials, for destruction or production of construction materials, and be mounted on corresponding equipment, intended for cutting and crushing of the above mentioned materials.
Generally, the cutting process mechanism is as shown in FIG. 1. Cutting of a material, like rock, is carried out by thrust force T and normal component C.sub.n of the cutting force, generated by an equipment drive. Under the action of these forces, the tool simultaneously moves in horizontal and vertical directions generating complicated stresses that overwhelm rock resistance.
Under the action of the force C.sub.n, transmitted by the bit front face, compressive stresses are formed in the rock which are not large enough for destruction but preload the rock to resist further strain.
Under the action of the force T, shear stress is produced in the rock due to the high level of load concentration created by the bit's cutting edge. This shear stress provokes generation and development of destructive cracks in the brittle material.
At the same time, both forces C.sub.n and T generate a confined zone of super pressurized rock, located next to the bit cutting edge. This so-called kernel is an accumulator of energy that can discharge in an explosive way when accumulated energy exceeds ultimate rock resistance.
Because the previously mentioned destructive cracks propagate from the cutting edge in the direction of lowest resistance, they initially tend toward the open surface of the rock. However, these cracks can not bypass the enhanced resistance of the volume of the rock compressed by C.sub.n. Consequently, the destructive cracks pass around the compressed rock and reach the open surface at a distance L from bit front face, isolating the stressed volume of rock and separating the chip from entire rock massif.
Under continuous combined action of compressive and shear stresses,successive rock chips are separated from the rock mass in a whole or nearly whole condition chiefly due to wide and active destructive cracks or by kernel explosion after sufficient energy is accumulated to overcome crack shortfall.
Therefore, in an effective rock cutting process, it is obligatory to maintain a significant load concentration at the bit cutting edge, which is provided by the positive rear angle .delta. of the bit.
Consequently, the effective cutting bit must have an optimal combination of high cutting ability and high durability of the cutting element, reliable protection from overloading, preservation of both a sharp cutting edge and the bit positive rear angle, and maintain other initial parameters throughout the life of the cutting bit.
A plurality of tools have been developed with the objective to achieve some of the above mentioned parameters. The first group of the tools are bits with non-rotatable cutting elements. U.S. Pat. No. 1,174,433 discloses a cutter with a convex front face; however, the angle between its longitudinal axis and the cut surface behind the bit (defined as the attack angle) is less than 90.degree. and the cutter is not protected from overloading or fast dulling of the cutting edge. U.S. Pat. Nos. 4,538,691 and 4,678,237 disclose destructive tools having elements with flat front face, oriented at a substantial negative front angle, that protects from overloading by providing a lifting force, but reduces the bit's cutting ability. The bits are not protected from fast dulling of the cutting edge. The attack angle here does not exceed 90.degree.. U.S. Pat. Nos. 4,538,690; 4,558,753; and 4,593,777 disclose bits with a concave front face, oriented at a large negative front angle, which provides protection from overloading but decreases bit cutting ability. The attack angle also does not exceed 90.degree.. The bit cutting edge is not protected from intensive dulling.
The second group of tools are rotative conical bits with a rock destructive element which can rotate around its own longitudinal axis. In the first sub-group of these tools, the cutting element has a conical shape (direct cone) and destroys the rock by its side surfaces, as disclosed for example in U.S. Pat. Nos. 3,650,565; 3,807,804; and 4,804,231.
These tools are bits of the crushing type that operate without generation of long destructive cracks. The bits are oriented at an attack angle which does not exceed 90.degree. and, as a rule, is no more than 60.degree. and bits are not protected from overloading. They have zero negative rear angle, their rotation around their longitudinal axis is not continuous and reliable. Therefore, their self-sharpening is not reliable, and if it occurs, the cutting element's initial angular parameters are not preserved.
The second group of the rotative bits includes tools which destroy rock with their end concave surfaces, as disclosed, for example, in U.S. Pat. No. 5,078,219. The bit here is a cutting tool, oriented with a small attack angle and is not protected from overloading. Its concave front face does not have a sufficient self-rotating and self-sharpening ability. Its rear angle has zero or negative value, and the bit quickly looses its cutting ability as it wears.